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G proteins, short for guanine nucleotide binding proteins, are a family of proteins involved in second messenger cascades. They are so called because of their signaling mechanism, which uses the exchange of guanosine diphosphate (GDP) for guanosine triphosphate (GTP) as a general molecular "switch" function to regulate cell processes. Alfred Gilman and Martin Rodbell were awarded the Nobel Prize in Physiology or Medicine in 1994 for their discovery of and research on G proteins.

G proteins are perhaps the most important signal transducing molecules in cells. In fact, diseases such as diabetes and certain forms of pituitary cancer, among many others, are thought to have some root in the malfunction of G proteins, and thus a fundamental understanding of their function, signaling pathways, and protein interactions may lead to eventual treatments and possibly the creation of various preventive approaches.

"G protein" usually refers to the membrane-associated heterotrimeric G proteins, sometimes referred to as the "large" G proteins. These proteins are activated by G protein-coupled receptors and are made up of alpha (α), beta (β) and gamma (γ) subunits.

There are also "small" G proteins or small GTPases like Ras that are monomeric, but also bind GTP and GDP and are involved in signal transduction.

Receptor-activated G proteins are bound to the inside surface of the cell membrane. They consist of the Gα and the tightly associated Gβγ subunits. When a ligand activates the G protein-coupled receptor, it induces a conformation change in the receptor (a change in shape) that allows the G protein to now bind to the receptor. The G protein then releases its bound GDP from the Gα subunit, and binds a new molecule of GTP. This exchange triggers the dissociation of the Gα subunit, the Gβγ dimer, and the receptor. Both, Gα-GTP and Gβγ, can then activate different signalling cascades (or second messenger pathways) and effector proteins, while the receptor is able to activate the next G protein. The Gα subunit will eventually hydrolyze the attached GTP to GDP by its inherent enzymatic activity, allowing it to reassociate with Gβγ and starting a new cycle.

A well characterized example of a G protein-triggered signalling cascade is the cAMP pathway. The enzyme adenylate cyclase is activated by Gαs-GTP and synthesizes the second messenger cyclic adenosine monophosphate (cAMP) from ATP. Second messengers then interact with other proteins downstream to cause a change in cell behavior.

G Protein

some hormones bind to protein receptors on the surface of the cell. the signal from the hormone is transmitted - or transduced - across the cell membrane by the receptor. Some hormone receptors then interact physically with G proteins on the intracellular side of the cell membrane and activate these G proteins. These G proteins are often comprised of three different peptide subunits called alpha, beta, and gamma G protein subunits. each subunit has a different function to transmit the hormonal signal to all the relative parts of the cell (nucleus, mitochondria, cytoplasm, vesicles, etc.).

there's a nice animation of how G proteins transmit the hormonal signal to the nucleus here: http://entochem.tamu.edu/G-Protein/index.html